Fluorescent carbon dot-based biosensor for the rapid and sensitive detection of Escherichia coli DNA

Rapid and accurate detection of pathogenic bacteria is critical for healthcare, food safety, and environmental monitoring. Here, we report a carbon dot-based fluorescent biosensor, prepared by a hydrothermal method, for sensitive and selective detection of Escherichia coli DNA. The carbon dots were characterized using field-emission scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, and energy-dispersive X-ray spectroscopy, confirming their uniform nanoscale morphology, structural integrity, and surface functionality. An amino-modified NH₂-BL21 DNA probe enabled specific hybridization with the target sequence, producing quantifiable fluorescence changes. The biosensor exhibited a linear detection range of 50-250 fM, a limit of detection of 38.36 fM, recoveries of 87.06-113.57%, and stable performance over four weeks, with minimal cross-reactivity toward non-target sequences. Moreover, the carbon dot-based biosensor accurately detected E. coli DNA in drinking water and tap water, with recoveries of 95-126 and 93-119%, respectively, for concentrations of 75-225 fM. Lake water recoveries ranged from 68 to 119% at low concentrations, indicating minor matrix effects. Unlike most previously reported carbon dot-based sensing strategies, which rely on enzymatic amplification or labeling, this work demonstrates an amplification-free and label-free fluorescence detection strategy that achieves femtomolar sensitivity through direct probe-target hybridization. The simple fabrication procedure, high repeatability, and robust analytical performance underscore the potential of this platform for the rapid, reliable, and cost-effective detection of Escherichia coli DNA. These findings highlight the promise of carbon dot-based fluorescent biosensors as practical tools for point-of-care diagnostics, environmental surveillance, and food safety monitoring.